Fix BOM breaking bin/*.bat and add early Graphviz check to --tosvg wrappers

setenv.bat (and activate_venv/get_cmd/install_py.bat) had a UTF-8 BOM,
which cmd.exe read as part of the @echo off line, breaking every bin
script that calls setenv.bat via `call`. Stripped the BOM.

material_flow.bat/.sh and tro_flow.bat/.sh now check up front whether
the Graphviz engine --tosvg needs (dot, or neato with --use-cords) is
resolvable, and print an early WARNUNG with fix instructions if not -
before the CSV is even parsed. The Python scripts still run and report
the same failure in detail if the SVG step is actually reached.

bin/tro_flow.bat

@@ -27,6 +27,26 @@ REM Python-Interpreter bestimmen (py-Launcher bevorzugt, sonst python)
 set "PY=py"
 py --version >nul 2>&1 || set "PY=python"
 
+REM Graphviz pruefen, wenn --tosvg SVG erzeugen soll (dot, bei --use-cords/
+REM --use-coords neato) - nur ein frueher Hinweis; das eigentliche Skript
+REM meldet denselben Fehler beim Rendern noch einmal im Detail.
+set "GV_ENGINE=dot"
+echo %*| findstr /i /c:"--use-cords" /c:"--use-coords" >nul && set "GV_ENGINE=neato"
+echo %*| findstr /i /c:"--tosvg" >nul
+if not errorlevel 1 (
+    where %GV_ENGINE%.exe >nul 2>&1
+    if errorlevel 1 (
+        set "GV_OK="
+        if /i "%GV_ENGINE%"=="dot" if defined GRAPHVIZ_DOT set "GV_OK=1"
+        if not defined GV_OK (
+            echo WARNUNG: Graphviz '%GV_ENGINE%' nicht im PATH gefunden - --tosvg wird vermutlich fehlschlagen.
+            echo          Graphviz installieren ^(https://graphviz.org/download/^), den bin-Ordner
+            echo          zum PATH hinzufuegen, oder GRAPHVIZ_DOT auf die dot.exe setzen ^(nur fuer 'dot'^).
+            echo.
+        )
+    )
+)
+
 "%PY%" "%SKEL_LIB%\tro_flow.py" %*
 set "RC=%ERRORLEVEL%"